The present invention relates an optical amplifying device, and more particularly to an optical amplifying device for amplifying a WDM (Wavelength Division Multiplex) optical signal.
Over the recent years, with developments of the Internet technologies, there has been a great leap in demands for information services, and a larger capacity and a more flexible network configuration are required of an optical transmission system in a backbone system.
There is WDM (Wavelength Division Multiplex) as a most effective transmission technology that responds to such a system demand. The WDM is defined as a transmission method of multiplexing fluxes of light having different wavelengths and simultaneously transmitting a plurality of signals via a single optical fiber, and its commercialization is now underway as centralized in North America.
On the other hand, an EDFA (Erbium-Doped Fiber Amplifier) is given as a key component for actualizing the WDM system. The EDFA is an optical amplifier capable of amplifying batchwise the wavelength-division-multiplexed optical signals by utilizing a wide gain band, wherein an Erbium (Er3+) doped fiber (EDF: Erbium-Doped Fiber) is used as a medium for amplification (the EDFA is capable of the wavelength-division-multiplexing amplification that is on the order of approximately 88 waves in, e.g., C-band).
An optical repeater for the WDM transmission generally takes a configuration in which the EDFAs are connected at multi-stages. Further, the EDFA has a gain wavelength characteristic. Therefore, it is of importance to flatten the gain wavelength characteristic within the signal band by equalizing the gains in a way that controls a sum of gains of the EDFAs at the respective stages into a fixed value in order to reduce both a scatter of peak power of each wavelength after the WDM transmission and S/N (Signal-to-Noise) deterioration.
FIG. 16 is a view showing a control image of the gain equalization. A gain wavelength characteristic (the same loss wavelength characteristic as) opposed to EDF gain wavelength dependency, is necessary for flattening the gain of an EDFA 101. Accordingly, the gain is flattened by providing a gain equalizer 102 having the same loss wavelength characteristic as the EDF gain wavelength dependency.
Further, FIG. 20 is an explanatory view in the case of controlling the gain equalization by the multi-staged EDFAs. For flattening the wavelength characteristic of a sum of gains of an EDF 111 and of an EDF 112, the gain is flattened by providing the gain equalizer 102 having a loss characteristic opposed to the gain wavelength dependency of the sum of the EDF gains.
On the other hand, if an optical input level, etc. of the optical repeater fluctuates with the result that the sum of gains of the multi-staged EDFs deviates from a fixed value, the flatness is not kept, and a tilt (gradient) occurs. This leads to decreases in transmission distance and in transmission band.
FIG. 17 is a view showing a result of measuring the fluctuation in the gain wavelength characteristic. Supposing that the 2-staged EDFs are installed in one single optical repeater, let A be a gain of one EDF and B a gain of the other EDF, and the axis of ordinates indicates the gain wavelength characteristic fluctuation (dB), while the axis of abscissa indicates the wavelength (nm).
An input signal to the optical repeater has, for instance, 8 waves equally allocated in signal wavelength bands 1575 nm through 1610 nm, and total power is changed between −14 dBm and −6 dBm (a dynamic range is 8 dB), wherein an amount of change is measured from the gain wavelength characteristic at −10 dBm.
In the case of controlling so that a sum of gains becomes a fixed value, i.e., A+B=k (fixed) at −10 dBm, the flatness is kept. In other cases where the gain tilt (gradient) is not compensated, however, when the input signal power to the optical repeater changes from −14 dBm to −6 dBm, it is understood that the gradient of the gain wavelength characteristic changes to “positive” from “negative”.
The occurrence of such a gain gradient induces deterioration of the transmission quality. Accordingly, the deterioration of the gain flatness has hitherto been compensated by performing the gain fixing control in a way that provides a variable optical attenuator (VOA) between the stages of the EDFs.
FIG. 18 is a diagram showing level diagrams of the optical amplification. Shown are optical levels of sections d1 through d3 in such a case that a VOA 114 is provided between the EDF 111 and the EDF 112 (an illustration of an optical pumping light source is omitted), and the optical signal is inputted from a left end of the EDF 111. Further, the level diagram when in a state 1 is depicted by a bold line, the level diagram when in a state 2 is drawn by a dotted line, and a level-overlapped portion is indicated by a fine solid line.
Supposing that A is the gain of the EDF 111 and B is the gain of the EDF 112, a desired amplifier gain G0 (a gain of the optical output) with no tilt shall be obtained when an attenuation quantity (a VOA set value) of the VOA 114 is v1 in the state 1. Thereafter, if the optical input level decreases from the state 1 down to the state 2, it is necessary to establish A+B=the fixed value in order not to cause the tilt in the desired amplifier gain G0. Hence, the relation “A+B=the fixed value” is kept as it is, and an amount of level fluctuation is adjusted by setting the VOA set value of the VOA 114 to v2 (the attenuation quantity is decreased), thereby obtaining the same amplifier gain G0 as in the state 1.
A conventional gain control using the VOA is that the deterioration of the gain flatness is compensated by providing a plurality of VOAs for every wavelength so as to fix the output level of the optical signal demultiplexed by a demultiplexer (e.g., Patent Document 1).
Further, as shown in FIG. 21, there is proposed a device contrived to obtain a fixed gain by applying gain fixing control at an input and an output of the EDFA (e.g., Patent Document 2). Moreover, as shown in FIG. 22, a method of applying the gain fixing control at each stage of the EDF (e.g., Patent Document 3).
Patent Document 1
Japanese Patent Application Laid-Open Publication No.2000-004062 (Paragraph No. [0020]–[0022], FIG. 1)
Patent Document 2
International Publication W001/005005 (pp. 11–12, FIG. 1)
Patent Document 3
Japanese Patent Application Laid-Open Publication No.8-248455 (Paragraph No. [0031]–[0032], FIG. 1)